US Defense Secretary Chuck Hagel got a first-hand look at China's new aircraft carrier on Monday as Beijing offered an unprecedented glimpse into its growing military clout.

Hagel praised the ship tour as a promising step forward for a fledgling US dialogue with the often secretive People's Liberation Army (PLA), but the two sides remained deeply divided on cyber-spying and regional territorial disputes.

"He understands how significant it was for the PLA to grant his request for a tour, and he was impressed by the professionalism of the officers and crew," Pentagon press secretary Rear Admiral John Kirby said.

"He hopes today's visit is a harbinger for other opportunities to improve our military-to-military dialogue and transparency," Kirby said.

Hagel's two-hour visit to the carrier in the port city of Qingdao kicked off a three-day trip to China, and officials said he was the first foreigner allowed aboard the vessel by the PLA.

His visit to China comes against the backdrop of friction between Washington and Beijing over cyber-spying, with each side trading accusations of digital espionage.

In a speech Tuesday at the PLA's National Defence University, Hagel planned to urge China to be more open about its cyber capabilities to defuse tensions and prevent a potential crisis, a senior defence official said.

- US shared cyber 'doctrine' -

"We have tried to be as open and transparent on that as we can be. And we would like to see them be able to reciprocate," said the official, who spoke on condition of anonymity.

"We very recently shared with them some basic doctrine on cyberspace, on how we're approaching the challenges in cyberspace," the official told reporters travelling with Hagel.

But the Chinese had not "reciprocated," said the official, confirming a New York Times report.

Last month the New York Times and Germany's Der Spiegel reported the US National Security Agency had hacked into the email archive of Chinese telecommunications giant Huawei, communications between top company officials and the source code of some of its products.

Washington has long seen Huawei as a security threat due to perceived close links to the Chinese government, which the company denies.

Despite friction over cyber security, officials said China's decision to invite Hagel aboard its sole aircraft carrier represented a solid step forward in US-China military relations.

The carrier, the Liaoning, symbolises China's ambition to project naval power far beyond its shores and reflects a military buildup that includes spending on sophisticated anti-ship missiles and submarines.

Reporters were not allowed to accompany Hagel to the carrier, but officials said the Pentagon chief and his delegation got a thorough tour of the ship, including a briefing from the carrier's skipper and a walk on the flight deck.

After the ship visit, some members of Hagel's staff were invited to fly from Qingdao to Beijing on a Chinese military cargo plane, officials said.

The Liaoning carrier was a Soviet-made warship based in Ukraine that the Chinese bought and refitted, completing the work in September 2012 in a milestone for the country's expanding military.

Beijing also has started constructing the second of four planned aircraft carriers, Chinese media reported in January.

Unlike American aircraft carriers, the Liaoning is not nuclear-powered and has a shorter range. Chinese officers openly acknowledged to the Americans the ship was not yet ready for combat and that training a corps of naval aviators would take some time.

"They know they have a long way to go with naval aviation," the defence official said.

The Liaoning nevertheless reinforces the image of China as a military power with global reach.

On Tuesday, Hagel is scheduled to meet his counterpart General Chang Wanquan in Beijing, two days after warning Beijing against unilateral attempts to settle territorial disputes with its smaller neighbours.

"You cannot go around and redefine boundaries, violate territorial integrity and sovereignty of nations by force, coercion and intimidation -- whether it's in small islands in the Pacific, or large nations in Europe," Hagel said in Tokyo, drawing a parallel with Russia's incursion into Ukraine.

"So I want to talk to our Chinese friends about this."

His comments underscored a tougher US line in recent months on China's approach to territorial disagreements in the South China Sea and the East China Sea, after some Southeast Asian countries accused Beijing of intimidatory tactics.